Andrew, I am not sure how it can be that you can play NTSC and PAL on a system without stuttering !?
Maybe your TV can better cope with that than mine ...
Did you change anything in Mythtv in the General page where one can setup different modes for GUI and TV playback - see attached picture
That could be a solution, but I did not get it working. It selects oright 1080i@50hz for LiveTV, but allways 1080p@60hz for mythvideo playback of DVD's. No matter if I did setup that it should use 1920x1080@50hz for 720x576 and 1920x1080@60hz for 720x480.
btw. in Linuxmce Video (xine) I still get some tearing. In Mythvideo picture is perfect using an GF 8400GS 512MB.
And one can change the frequncy using xrandr :
xrandr -s 1920x1080 -r 60
xrandr -s 1920x1080 -r 50
If you do it using a ssh connection, add "-d 0:0" to th line.
Just an idea, that could togehter with the above mplayer detection (or some thing else) be used to setup the right frequency for the video that we will be playing...